This invention relates to a furnace for baking molded carbon shapes. More particularly, it relates to the refractory design and construction of ring-type furnaces for baking carbon anode and cathode blocks used in an electrolytic process for making a metal such as aluminum, for example.
Ring furnaces for baking carbon anode and cathode blocks used to produce aluminum are well known. A ring furnace is constructed in a manner which enables sequential preheating, baking, and cooling molded carbon blocks held in chambers commonly called pits on a continuous basis. The progression of these sequential operations is enabled by the induced flow of flue gases, fuel, and combustion/cooling air in a closed rectangular loop or ring of furnace flues adjacent the pits; hence, the name ring furnace.
The flues are formed by long parallel rows of spaced apart refractory end-to-end fluewalls, with the row ends joined together by a common flue passage called a crossover. A typical ring furnace has from 12 to 24 parallel rows of flues, and two such crossovers. Half of the parallel flue rows reside in the north (or east) side of a ring furnace and the other half reside in the south (or west) side of the furnace. Flue gas flow is in one direction through one side and in the opposite direction through the other side, the flow loop being closed by the common crossover flue at each end of the furnace. The inner flue row on each side of a furnace is typically spaced 2 to 10 feet away from the furnace centerline, depending on the furnace cranes and building design utilized.
The parallel rows of flues within each half furnace are spaced apart uniformly to form the sidewalls of open-top pits into which the carbon anode or cathode blocks are placed for baking. Pit width, depth and length are sized to efficiently accommodate the carbon blocks to be baked. Flue length and depth are conformed to pit length and depth. Flue widths typically have ranged from 15.75 to 20.25 inches. Pit and flue sizes typically are constant within a furnace, but differ from furnace to furnace. To form the end walls of individual pits and to interlock adjoining ends of fluewalls in each long row of flues, refractory headwalls are constructed laterally across each half furnace, at intervals determined by the desired pit length. In ring furnaces as built heretofore, the headwall width has typically been 18 inches between pits and 9 inches between the butting ends of flues where the fluewall ends fit into 4.5 inch deep vertical recesses (slots) on each side of the headwall. The lateral assembly of pits and fluewalls contained between successive headwalls in each half furnace is typically called a furnace section. Each section typically contains 5 to 11 pits and 6 to 12 flues. Each half furnace typically contains 16 to 48 sections (32 to 96 sections per furnace). The number of pits and flues per furnace section, and the number of sections per furnace, are a function of the output of baked blocks required from the furnace.
In operation of such a carbon baking furnace, all pits in a given section are at the same stage in the baking cycle at any given time. Sections are loaded and paced through the baking cycle in succession in a given direction, either clockwise or counterclockwise, around the furnace. At any given time, some sections of pits will be empty, some will be receiving their next loading of carbon blocks and packing coke, some will be heating, some soaking at final temperature, some cooling, some being unloaded, and some being repaired (reconditioned) prior to being reloaded for their next baking cycle. This operating cycle is imposed on each section of pits by a systematic repositioning of furnace firing equipment from section to section, at a specified frequency. The firing equipment consists of fabricated assemblies which rest on top of the furnace and typically are movable by overhead crane. The assemblies function to input fuel, input cooling and combustion air, exhaust spent flue gases, and control flue gas pressure and/or fluewall temperature. Each baking furnace typically has sufficient furnace sections for operation of multiple (usually 2 to 4) simultaneous baking cycles. Each baking cycle typically requires 16 to 26 tandem sections, the exact number being a function of the intended operating plan and expected pit productivity. Thus, a furnace for two simultaneous baking cycles, with 16 sections per cycle, would contain 32 furnace sections.
To complete each baking cycle, furnace refractories must be cycled through a wide temperature range. Fluewall temperature fluctuates from a low near room temperature to a high of 1250.degree.-1350.degree. C., and back to the low. Headwalls are cycled through only a slightly lower temperature range. The temperature changes induce commensurate expansion-contraction reversals which cause movement, and shifting, in both the fluewalls and headwalls. Space for the expansion must be provided at the ends of each fluewall and at intervals within, or at the ends of, each headwall. The major headwall expansion is lateral (at 90.degree.) to the direction of major fluewall expansion. In the past, this relative movement, and other factors such as in-service shrinkage within the refractories, results with time in an ever-increasing looseness of fit of fluewall to headwall at each pit corner. Yet this fit, between each pit face of a fluewall and the adjacent side face of the headwall recess, must be kept "coke-tight" to prevent leakage of packing coke from the pits to the recess then into the flues. The coke is in loose powder form and is placed around and on top of the carbon blocks in each pit to prevent carbon oxidation (air-burning) and conduct heat to and from the blocks. Loss of coke into the flues can restrict the flue passage and reduce combustion efficiency within the flues and heat transfer between flues and pits. Entrained in flue gases, coke dust may create a fire hazard in the exhaust system and/or an emissions problem. Within the flues, it can burn out of control, causing localized overheating which distorts the fluewalls. Flues may also become bowed due to loss of expansion space in headwall recesses if the recesses are filled with coke.
It is essential, therefore, that the seal at the pit corners is maintained; and to do so has heretofore required the employment of expensive repair procedures. Because of the severity of the problems engendered by pit corner breakdown, it would be highly desirable to provide a furnace structure which maintains a tight seal at pit corners for relatively long periods of furnace operation.